Return-Path: Received: from fieldses.org ([174.143.236.118]:36398 "EHLO fieldses.org" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1758119Ab0LNWBM (ORCPT ); Tue, 14 Dec 2010 17:01:12 -0500 Date: Tue, 14 Dec 2010 17:01:02 -0500 From: "J. Bruce Fields" To: Nick Piggin Cc: Alexander Viro , Nick Piggin , linux-nfs@vger.kernel.org, linux-fsdevel@vger.kernel.org Subject: Re: [PATCH] nfsd4: allow __d_obtain_alias() to return unhashed dentries Message-ID: <20101214220102.GM24828@fieldses.org> References: <20101112184353.GA32745@fieldses.org> <20101115174837.GB10044@fieldses.org> <20101129193248.GA9897@fieldses.org> <20101203223326.GB28763@fieldses.org> <20101213051944.GA8688@amd> Content-Type: text/plain; charset=utf-8 In-Reply-To: <20101213051944.GA8688@amd> Sender: linux-nfs-owner@vger.kernel.org List-ID: MIME-Version: 1.0 On Mon, Dec 13, 2010 at 04:19:44PM +1100, Nick Piggin wrote: > On Fri, Dec 03, 2010 at 05:33:27PM -0500, J. Bruce Fields wrote: > > From: J. Bruce Fields > > > > Without this patch > > > >        client$ mount -tnfs4 server:/export/ /mnt/ > >        client$ tail -f /mnt/FOO > >        ... > >        server$ df -i /export > >        server$ rm /export/FOO > >        (^C the tail -f) > >        server$ df -i /export > >        server$ echo 2 >/proc/sys/vm/drop_caches > >        server$ df -i /export > > > > the df's will show that the inode is not freed on the filesystem until > > the last step, when it could have been freed after killing the client's > > tail -f.  On-disk data won't be deallocated either, leading to possible > > spurious ENOSPC. > > > > This occurs because when the client does the close, it arrives in a > > compound with a putfh and a close, processed like: > > > >        - putfh: look up the filehandle.  The only alias found for the > >          inode will be DCACHE_UNHASHED alias referenced by the filp > >          associated with the nfsd open.  d_obtain_alias() doesn't like > >          this, so it creates a new DCACHE_DISCONECTED dentry and > >          returns that instead. > > > > Nick Piggin suggested fixing this by allowing d_obtain_alias to return > > the unhashed dentry that is referenced by the filp, instead of making it > > create a new dentry. > > > > Cc: Nick Piggin > > Signed-off-by: J. Bruce Fields > > --- > > fs/dcache.c | 2 +- > > 1 files changed, 1 insertions(+), 1 deletions(-) > > > > On Tue, Nov 30, 2010 at 12:00:16PM +1100, Nick Piggin wrote: > > > On Tue, Nov 30, 2010 at 6:32 AM, J. Bruce Fields wrote: > > > > On Mon, Nov 29, 2010 at 02:56:22PM +1100, Nick Piggin wrote: > > > >> On Tue, Nov 16, 2010 at 5:45 PM, Nick Piggin wrote: > > > >> > On Tue, Nov 16, 2010 at 4:48 AM, J. Bruce Fields wrote: > > > >> >> On Sat, Nov 13, 2010 at 10:53:12PM +1100, Nick Piggin wrote: > > > >> >>> Can you even put the link check into __d_find_alias? > > > >> >>> > > > >> >>> -               if (S_ISDIR(inode->i_mode) || !d_unhashed(alias)) { > > > >> >>> +               if (S_ISDIR(inode->i_mode) || !inode->i_nlink || > > > >> >>> !d_unhashed(alias)) { > > > >> >>> > > > >> >>> Something like that? > > > >> >> > > > >> >> The immediate result of that would be for the close rpc (or any rpc's > > > >> >> sent after the file was unlinked) to fail with ESTALE. > > > >> > > > > >> > Why is that? Seems like it would be a bug, because a hashed dentry may > > > >> > be unhashed at any time concurrently to nfsd operation, so it should be > > > >> > able to tolerate that so long as it has a ref on the inode? > > > >> > > > >> Ping? Did you work out why nfs fails with ESTALE in that case? It seems > > > >> to work in my testing (and do the right thing with freeing the inode). > > > > > > > > Bah, sorry, I read too quickly, got the sense of the test backwards, and > > > > thought you were suggesting __d_find_alias() shouldn't return an alias > > > > in the i_nlink == 0 case! > > > > > > > > Yes, agreed, that should solve my problem. > > > > > > OK, good. > > > > > > > But what's the reason for the d_unhashed() check now?  Could we get rid > > > > of it entirely? > > > > > > Well when the inode still has links I think we actually do want any new > > > references to go to hashed dentries. Definitely for d_splice_alias. > > > > So here's a version with a changelog; objections? > > Not sure where Al's hiding... > > But I would like to update the comments, and perhaps even a new > add a new function here (or new flag to __d_find_alias). > > AFAIKS, the callers are OK, however I suppose d_splice_alias and > d_materialise_unique should not have unlinked inodes at this point, > so at least a BUG_ON for them might be a good idea? That does sound safer. I'm pretty confused by the various __di_splice_alias callers. I'll go search through them and see if I can understand better.... --b. > > > > > --b. > > > > diff --git a/fs/dcache.c b/fs/dcache.c > > index 23702a9..afa8a0d 100644 > > --- a/fs/dcache.c > > +++ b/fs/dcache.c > > @@ -368,7 +368,7 @@ static struct dentry * __d_find_alias(struct inode *inode, int want_discon) > > next = tmp->next; > > prefetch(next); > > alias = list_entry(tmp, struct dentry, d_alias); > > - if (S_ISDIR(inode->i_mode) || !d_unhashed(alias)) { > > + if (S_ISDIR(inode->i_mode) || !inode->i_nlink || !d_unhashed(alias)) { > > if (IS_ROOT(alias) && > > (alias->d_flags & DCACHE_DISCONNECTED)) > > discon_alias = alias;